First of all, get a copy of the Mythical Man Month written by Frederick Brooks. It is 50 years old, but it prooves that our profession is still repeating the same mistakes that our parents or grand-parents were doing.
The main issue isn't the 400 hours you get, it is the x hours you loose by managing the other person. In my experience it takes you at least a month and the other person six months to become fully productive. You have a third, so you are basically wasting over 400 hours. Make sure you waste them efficiently, so it doesn't waste a lot of your time. Testing, documentation, research so unds like a good plan for you, though not the other side.
Obviously, it would be better if you could convince those responsible of a long term contract, maybe at 20 hours a week instead of 40.
The biggest issue of so source control is again not setting things up. That is a matter of a few weeks if you start clueless and have to migrate an average project. The difficult part is providing solutions when you encounter a problem, like having screwed up in a merge conflict and neeeding someone to clean up the mess.
Subversion is relatively outdated. I've talked about SVN at conferences almost a decade ago. Today I would look at Git or Mercurial...